The effects of ionizing and ultra-violet radiation on nucleic acids and proteins and their constituents are being studied. The modification of radiation damage in DNA by cancer chemotherapy agents of the intercalating and alkylating types is of interest since such information may be useful in radiation therapy. The present report includes electron spin resonance (ESR) studies of spin-trapped free radicals generated in the photochemistry of Daunomycin and Adriamycin in connection with the possible use of these drugs in photoradiation therapy of tumors. At 438 nm the formation of superoxide anion radicals in aerated aqueous solutions was established. Mechanistic studies indicated the absence of hydrated electrons, hydroxyl radicals and of singlet oxygen. The formation of superoxide anion radicals occurs either by the reaction of quinone anion radicals or of the quinone excited states with oxygen. Since the presence of hydroxyl anion radicals in non-irradiated biological systems in which superoxide anion radicals are formed by metabolic processes has been investigated by many investigators by spin-trapping methods, we have examined the quantitative aspects of this technique using Gamma-irradiated aqueous solutions where radical yields are accurately known. For hydroxyl radicals the spin trapping efficiency for the spin trap 5,5-dimethyl-1-pyrroline N-oxide N-oxide (DMPO) is about 35%. The quantitative aspects of determining free radicals in polycrystalline amino acids Gamma-irradiated at room temperature and subsequently dissolved in spin-trap solutions were also investigated. We have applied the new Resolution Enhanced ESR method (2nd harmonic, out of phase) to the study of nitroxide radicals. With this method in conjunction with computer simulation, we could obtain superhyperfine coupling constants for spin-labels and for the spin adducts of several spin traps. It was possible to distinguish radicals whose spin adducts would be identical by conventional ESR. Continuation of our studies of the effects of ultrasound on aqueous solutions showed the formation of hydroxyl radicals and hydrogen atoms in the collapsing cavitation bubbles produced by typical dental equipment.